The present invention relates to a cylinder piston apparatus used in a car suspension system, and more specifically, to the improvement of a cylinder piston apparatus having an oil chamber and a damping-force generating mechanism therein.
Prior art cylinder piston apparatuses for suspension systems, which are adapted to contain oil and gas, comprise a cylinder housing and a rod fitted therein. An oil chamber and a gas chamber are defined inside the cylinder housing, as well as a damping-force generating mechanism having an orifice. The reciprocation of the rod, relative to the cylinder housing, is damped by viscous resistance which is produced as the oil passes through the orifice.
Cylinder piston apparatuses of this type have conventionally been proposed whose damping force can be varied. Examples of these apparatuses include shock absorbers which are disclosed in U.S. Pat. Nos. 4,527,676 and 4,620,619. In these apparatuses, a rotary valve is fitted in a valve housing which is attached to the damping-force generating mechanism. The rotary valve is driven by a motor. The valve housing has a plurality of first circulation holes of different sizes, while the rotary valve has a plurality of second circulation holes. When the rotary valve is rotated to a predetermined position, the first and second circulation holes communicate with one another, so that a damping force can be produced, which corresponds to the size of the circulation holes.
The motor is located in a motor-holding chamber which is defined inside the rod. Conventionally, the holding chamber is isolated from the oil chamber by means of a seal member, such as an O-ring, in order to keep the motor from coming into contact with the oil. To prevent the oil in the oil chamber from flowing into the motor-holding chamber, therefore, the output shaft of the motor is also sealed by means of a seal member.
In the prior art apparatuses constructed in this manner, the rotation of the motor shaft is impeded by friction generated between the shaft and the seal member. When the oil flows through the damping-force generating mechanism, moreover, a differential pressure is produced between the inside and outside of the rotary valve. Accordingly, if the motor used is a low-torque motor, such as a stepping motor, the rotary valve sometimes cannot rotate smoothly, and is thus unable to change the damping force accurately.